Dissolved halogen detectors are primarily designed for the detection of chlorine, but will also respond to bromine and iodine. Two detection methods are generally used: potentiometric and amperometric. Potentiometric electrodes respond to total oxidizers and are based upon the development of a potential at an indicating or oxidation-reduction potential electrode which is usually constructed of platinum (OPR) electrode). Platinum ORP electrodes have been used in combination with a reference electrode in certain harsh chemical environments, e.g., hot brine streams, but have been prone to early failure due to corrosion and fouling.
Amperometric halogen electrodes are based upon the principle that halogens are easily reduced when the proper potential is applied to an electrode. A current will flow which is proportional to the concentration of the halogen. Two electrodes are required, one being the cathode (working) electrode, often of platinum, at which the halogen is reduced, and the other being the anode (counter) electrode at which some other reaction occurs to provide the necessary current flow. Premature electrode corrosion and fouling occur with amperometric electrodes, similarly as with the described platinum ORP electrodes.